Deposits of oil sands are found around the world, but most prominently in Canada, Venezuela, and the United States. These oil sands contain significant deposits of heavy oil, typically referred to as bitumen. The bitumen from these oil sands may be extracted and refined into synthetic oil or directly into petroleum products.
The difficulty with bitumen lies in that it typically is very viscous, sometimes to the point of being more solid than liquid. Thus, bitumen typically does not flow as less viscous, or lighter, crude oils do.
Because of the viscous nature of bitumen, it cannot be produced from a well drilled into the oil sands as is the case with lighter crude oil. This is so because the bitumen simply does not flow without being first heated, diluted, and/or upgraded. Since normal oil drilling practices are inadequate to produce bitumen, several methods have been developed over several decades to extract and process oil sands to remove the bitumen. For shallow deposits of oil sands, a typical method includes surface extraction, or mining, followed by subsequent treatment of the oil sands to remove the bitumen.
The development of surface extraction processes has occurred most extensively in the Athabasca field of Canada. In these processes, the oil sands are mined, typically through strip or open pit mining with draglines, bucket-wheel excavators, and, more recently, shovel and truck operations. The oil sands are then transported to a facility to process and remove the bitumen from the sands. These processes typically involve a solvent of some type, most often water or steam, although other solvents, such as hydrocarbon solvents, have been used.
After excavation, a hot water extraction process is typically used in the Athabasca field in which the oil sands are mixed with water at temperatures ranging from approximately 35° C. to 75° C., with recent improvements lowering the temperature necessary to the lower portion of the range. An extraction agent, such as sodium hydroxide (NaOH), surfactants, and/or air may be mixed with the oil sands.
Water is added to the oil sands to create an oil sands slurry, to which additives such as NaOH may be added, which is then transported to an extraction plant, typically via a pipeline. Inside a separation vessel, the slurry is agitated and the water and NaOH releases the bitumen from the oil sands. Air entrained with the water and NaOH attaches to the bitumen, allowing it to float to the top of the slurry mixture and create a froth. The bitumen froth is further treated to remove residual water and fines, which are typically small sand and clay particles. The bitumen is then either stored for further treatment or immediately treated, either chemically or mixed with lighter petroleum products, and transported by pipeline for upgrading into synthetic crude oil. Unfortunately, this method cannot be used for deeper tar sand layers. In situ techniques are necessary to recover deeper oil in well production. It is estimated that around 80 percent of the Alberta tar sands and almost all of the Venezuelan tar sands are too far below the surface to use open pit mining.
In well production, referred to as in situ recovery, Cyclic Steam Stimulation (CSS) is the conventional “huff and puff” in situ method whereby steam is injected into the well at a temperature of 250° C. to 400° C. The steam rises and heats the bitumen, decreasing its viscosity. The well is allowed to sit for days or weeks, and then hot oil mixed with condensed steam is pumped out for a period of weeks or months. The process is then repeated. Unfortunately, the “huff and puff” method requires the site to be shut down for weeks to allow pumpable oil to accumulate. In addition to the high cost to inject steam, the CSS method typically results in 20 to 25 percent recovery.
Steam Assisted Gravity Drainage (SAGD) is another in situ method where two horizontal wells are drilled in the tar sands, one at the bottom of the formation and another five meters above it. The wells are drilled in groups off of central pads. These wells may extend for miles in all directions. Steam is injected into the upper well, thereby melting the bitumen which then flows into the lower well. The resulting liquid oil mixed with condensed steam is subsequently pumped to the surface. Typical recovery of the available oil is 40 to 60 percent.
The above methods have many cost, environmental and safety problems associated with them. For example, the use of large amounts of steam is energy intensive and requires the processing and disposal of large amounts of water. Currently, tar sands extraction and processing requires several barrels of water for each barrel of oil produced. Strip mining and further treatment results in incompletely cleaned sand, which requires further processing, before it can be returned to the environment. Further, the use of a large quantity of caustic in surface mining not only presents process safety hazards but also contributes to stability of fine clay particles in tailings, the disposal of which is a major environmental problem.
Thus, there remains a need for efficient, safe and cost-effective methods to improve the recovery of bitumen from oil sands.